Re: [time-nuts] Distribution amplifier for 10MHz and 1 pps

[Perry Sandeen via time-nuts]

Yo Bubba Dudes!
Wrote: 
Gosh. This topic comes up at least once every 6 months. Adiligent search of the 
time-nuts database would turn up amazing things. . .
Yep, it does and it always will for at least two reasons.
One, we have new people joining the list.  Which is great. They're at the 
beginning of a learning curve and most aren't in the upper echelons of TN 
knowledge.  It's a hobby for them not an extension of their professional lives. 
 But they want to learn and make progress

New members aren't aware of the past data base of info, and speaking for myself 
I never found an easy way to search.  If there is an easy way to do so I'm 
willing to be taught.
I learn a lot from the posters even if it is far above my technical skills.  If 
I have no understanding or interest of post(s), I just delete it and keep what 
interests or is helpful to me.
That's as it should be, and hopefully always will be, on a list like this with 
such a diverse mix of members.
Now we all started spending 9 months in a nice warm water bath happily sucking 
our thumb.
Then some dude takes us out, turns us upside down, and slaps us on our butt 
while saying *Welcome to the world*!
Now what we think is: Say what sucker! I was perfectly happy where I was and 
minding by own business.
However, like it or not, we are then at the start of the unending learning 
curve until our lights goes out figuratively or literally.
Regards,
Perrier








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[time-nuts] inexpensive fiber optic distribution

[jimlux]

While researching this for a radio telescope, I found amazingly 
inexpensive (<$10) fiber optic transceivers for 1Gbps sorts of rates..


They take a standard singlemode duplex fiber and have a pinout suitable 
for plugging into a Cisco, etc. switch. Apparently, there's some 
internal programming which sets the hardware for the kind of switch, 
rates, etc..  fs.com has a generic ASIC inside the adapter which can be 
reprogrammed for whatever formatting, compatibility etc is needed. The 
astronomers were saying that at Berkeley, they bought a box for a few k 
which allows them to reconfigure the transceivers.



Something like this:
https://www.fs.com/products/12622.html

Data sheet here: https://img-en.fs.com/file/datasheet/sfp1g-lx-31-10km.pdf

and a 10 meter patch cable is $5
https://www.fs.com/products/40203.html

This is sort of "old news" in the networking world (the SFP form factor 
was defined in 2001)



It seems like you could probably figure out how to interface to these 
things and use them to distribute timing signals.


I'll be talking to the radio astronomers tomorrow again, and I'll see 
what I can find out about interfaces. I suspect they do NOT use them for 
conventional networking - they're all about running lots of bits from 
antennas into large multichannel correlators/beamformers.


They're buying these things by the dozen.

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Re: [time-nuts] Yet another GPSDO project.

[Bob kb8tq]

Hi

One nice thing about just getting the TICC from TAPR:

https://www.tapr.org/kits_ticc.html 

Is that all of the glue and hook it up stuff has already been done and debugged.
There’s also code on the attached MCU to take care of all sorts of odd stuff.

John has a nice writeup at:

https://www.tapr.org/pdf/DCC2016-The_TAPR_TICC_Counter_Measuriing_Trillionths_of_a_Second_with_an_Arduino_John_Ackerman_N8UR.pdf
 


Since you are after a continuously running count (and not the difference 
between readings) the builtin time tagging software does just fine in this 
case. 
If you want to go full DIY, I think you probably are better of grabbing one of 
the cheap FPGA boards off eBay and doing things up in it. A Max 10 from Altera 
/ Intel is < $50. It will PLL a 10 MHz clock up to 400 MHz. Programming and 
design software is free. Data entry into the design side can be via a 
schematic. No need to learn Verilog.

Bob



> On Mar 20, 2019, at 4:58 PM, Tobias Pluess  wrote:
> 
> Hi again Bob,
> 
> I read some documentation about your TICC and tried to figure out how I could 
> implement something similar in my GPSDO, but having only one channel. I ended 
> up with this:
> 
> https://hb9fsx.ch/wordpress/wp-content/uploads/2019/03/20190320_214120.jpg
> 
> It works as follows. The GPS 1PPS is asynchronous to the 10MHz clock. So it 
> occurs anywhere. It is synced to the 10MHz and delayed by 300ns, the same as 
> in the TICC. The rising edge of the 1PPS is used as the start signal for the 
> TDC7200, and the rising edge of the delayed 1PPS is used as the stop signal.
> 
> While all this happens, the counter inside the microcontroller counts, and is 
> latched when the microcontroller detects the rising edge.
> 
> On the next 1PPS signal from the GPS the same things happen again, and and 
> the counter is latched another time.
> In my example, the counter counts from 00 to 08, and the TDC7200 gives the 
> additional fractional 70ns.
> What do you think?
> 
> It needs not 2 TDCs but only one, but it also requires quite a bit of 
> external glue logic. And I am not sure about the delay of 300ns: if the 
> propagation delay of the logic producing this delay is not accurately known, 
> the delay is not 300ns but 300ns + some offset, isn't it?
> 
> 
> Tobias
> HB9FSX
> 
> 
> From: time-nuts [time-nuts-boun...@lists.febo.com] on behalf of Bob kb8tq 
> [kb...@n1k.org]
> Sent: Wednesday, March 20, 2019 14:27
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Yet another GPSDO project.
> 
> Hi
> 
> Indeed you want a digital “phase detector” of some sort. Your counter on the 
> micro
> is a phase detector. If you clock it off of the OCXO and read it based on the 
> PPS,
> what you get is phase. If you never arbitrarily reset the running counter, it 
> reads phase
> forever and ever (no slips). If you get a micro that lets you phase lock the 
> internal clock,
> you can come up with a counter that runs in the hundreds of MHz. That gets 
> you at least
> into the 10’s of ns. An FPGA is another way to get a counter like this.
> 
> Since things like counters are stable long term, they make good phase 
> detectors in a
> case like this. You might “help” a little to get the last few digits (as the 
> TICC does). Still
> the bulk of what you read is done by a simple counter. Drift is still taken 
> out and you
> hang on to your phase reference long term.
> 
> As a very simple example:
> 
> Most micro’s will let you set up a counter so it rolls over at a set point. 
> The example
> below assumes it counts to some yy….. = one second value and the next 
> count
> is zero.
> 
> Second by second my counter at the PPS edge reads out:
> 
> 1204
> 1212
> 1223
> 1232
> 1231
> 1245
> 1259
> …..
> 1325
> 
> and on it goes.
> 
> Since I never reset the count, it’s off by some weird amount. I really don’t 
> care what that is.
> I can look at the readings and if the first and last are 10 seconds apart 
> that’s phase over 10
> seconds. In the example above my phase changed by 1325-1204 = 121 counts.
> 
> All the loop needs to do is to “steer” things so the count stays at xxx1204. 
> Subtract what I
> saw when I first started up from whatever I got just now and keep that at 
> zero.
> 
> Yes this all *assumes* the counter has been set up to roll over nicely at the 
> one second
> transition. If not, there is a bit more math to do to account for that issue. 
> There are also
> gotcha’s with counter / timer’s on micro’s maybe not doing quite what you 
> think they should
> due to clock domain issues.
> 
> Some designs out there run a very small counter ( = ignore all the  in 
> the example above)
> and *assume* that things will never roll over. There are lots of 

Re: [time-nuts] Yet another GPSDO project.

[Jim Harman]

Tobias,

If your controller has a 10 bit ADC available, I would suggest using the
TIC circuit in Lars's GPSDO. For a 10 MHz input, it needs only a 74HC390
divide by 10, a 74HC4046 phase detector, plus 1 diode, 1 capacitor, and 2
resistors to produce a 1 nsec resolution TIC. You get a fresh TIC value
every second and the phase detector has a lock-in range of 1 usec.

. 


-- 

--Jim Harman
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Re: [time-nuts] Yet another GPSDO project.

[Tobias Pluess]

Hi Bob

yes, this is exactly what I do currently.
My microcontroller offers some 32 bit counters. I use the OCXO as clock for one 
of these and let the counter run all the time, it is never stopped. With every 
rising edge of the 1PPS signal, the counter value is copied into some register 
(input capture) while the counter continues to run.
The cool thing is: if you interpret the value read from the counter as a signed 
32 bit integer, you don't have to care about overflows. It works if one always 
subtracts the previous reading from the current one!
However, the resolution is only 100ns.
The microcontroller offers a PLL; I tried to use that, and the datasheet says 
it has a jitter of 300ps - sounds good to me. However, when I used the PLL, the 
frequency stability was quite bad. Thanks to a post in the EEVBlog forum I 
found out that the PLL has some sort of frequency modulation or sawtooth 
waveform, which basically renders it useless for this kind of application, 
unfortunately :-(
But indeed, having a PLL would be great because this particular microcontroller 
can run with up to 60MHz. So if I used, say 50MHz, the resolution could easily 
be increased to 20ns, which I think is way better than what I currently have.

Of course the resolution could be increased greatly if I combined this approach 
with a TDC7200, as you have used one in your TICC. However I am unsure whether 
this would work as intended. The TDC7200 needs a reference clock. Of course the 
OCXO would be ideal for it - but as long as the GPSDO is not locked, the OCXO 
frequency will be wrong by some (unknown) amount, and thus the time measurement 
of the TDC7200 will be wrong as well, won't it?
I am currently trying to figure out how I may connect the TDC7200 to my 
hardware.

However it sill does not solve the problem that the phase of my OCXO is still 
quite arbitrary and not aligned to UTC and so is my 1PPS output.

How actually do professional GPSDOs work? in my STAR4 GPSDO, there is only one 
large FPGA which does the magic - I am pretty sure they use a more 
sophisticated algorithm than I do.

Best
Tobias


From: time-nuts [time-nuts-boun...@lists.febo.com] on behalf of Bob kb8tq 
[kb...@n1k.org]
Sent: Wednesday, March 20, 2019 14:27
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Yet another GPSDO project.

Hi

Indeed you want a digital “phase detector” of some sort. Your counter on the 
micro
is a phase detector. If you clock it off of the OCXO and read it based on the 
PPS,
what you get is phase. If you never arbitrarily reset the running counter, it 
reads phase
forever and ever (no slips). If you get a micro that lets you phase lock the 
internal clock,
you can come up with a counter that runs in the hundreds of MHz. That gets you 
at least
into the 10’s of ns. An FPGA is another way to get a counter like this.

Since things like counters are stable long term, they make good phase detectors 
in a
case like this. You might “help” a little to get the last few digits (as the 
TICC does). Still
the bulk of what you read is done by a simple counter. Drift is still taken out 
and you
hang on to your phase reference long term.

As a very simple example:

Most micro’s will let you set up a counter so it rolls over at a set point. The 
example
below assumes it counts to some yy….. = one second value and the next count
is zero.

Second by second my counter at the PPS edge reads out:

1204
1212
1223
1232
1231
1245
1259
…..
1325

and on it goes.

Since I never reset the count, it’s off by some weird amount. I really don’t 
care what that is.
I can look at the readings and if the first and last are 10 seconds apart 
that’s phase over 10
seconds. In the example above my phase changed by 1325-1204 = 121 counts.

All the loop needs to do is to “steer” things so the count stays at xxx1204. 
Subtract what I
saw when I first started up from whatever I got just now and keep that at zero.

Yes this all *assumes* the counter has been set up to roll over nicely at the 
one second
transition. If not, there is a bit more math to do to account for that issue. 
There are also
gotcha’s with counter / timer’s on micro’s maybe not doing quite what you think 
they should
due to clock domain issues.

Some designs out there run a very small counter ( = ignore all the  in the 
example above)
and *assume* that things will never roll over. There are lots of alternatives …

Bob



> On Mar 20, 2019, at 3:53 AM, Tobias Pluess  wrote:
>
> Hi,
>
> no, I don't want the OCXO to closely follow the 1PPS - this would make no 
> sense. Instead I thought the "right way" of doing it is perhaps:
>
> a) measure the phase difference between the GPS PPS and the PPS derived from 
> the OCXO.
> b) put that phase difference into a loop filter.
> c) the loop filter's output goes to a DAC.
>
> Of course I don't want to have an analog phase detector, nor an analog loop 
> 

Re: [time-nuts] Yet another GPSDO project.

[Tobias Pluess]

Thanks Achim :-)
I actually have no idea what foam it is *exactly*, but I would call it Sagex, 
though, because it is somewhat similar.
But it is mich lighter and quite hard, so it is not exactly Sagex, you are 
right! I used this name just because I lacked a better one.

My GPS module indeed has both EXTINT pins available. I just read about the 
"Aiding" topic; it says that one can provide an external rectangular signal on 
one of the EXTINT pins for what is called "frequency aiding".

"Frequency aiding can be implemented by connecting a periodic rectangular 
signal with a frequency up to
500 kHz and arbitrary duty cycle"

However I didn't understand what I gain from this - does it discipline the TCXO 
inside the GPS module?
But I think I won't get better phase resolution this way, I guess.

" You'll end up with some FLL/PLL hybrid most likely."
perhaps. What I have now is just an FLL, I think.

Best
Tobias



From: time-nuts [time-nuts-boun...@lists.febo.com] on behalf of Achim Gratz 
[strom...@nexgo.de]
Sent: Wednesday, March 20, 2019 19:14
To: time-nuts@lists.febo.com
Subject: Re: [time-nuts] Yet another GPSDO project.

Tobias Pluess writes:
> By the way, this is the unit I built:
>
> https://hb9fsx.ch/wordpress/index.php/2018/11/14/my-own-gpsdo-part-i/

Nice build.  Minor nit: Whatever foam you used to isolate your OCXO
further is probably not SAGEX, but looks to be either cross-linked PE
foam or (more likely) PU foam.  Isolating the unit like that shifts down
the maximum usable ambient temperature and may confuse the temperature
controller if you overdo it.

> I have also a homemade antenna for it, since I read somewhere that the
> quadrifilar helix antennas are better than patch antennas, I made one
> by myself. A choke ring antenna would perhaps be even more awesome,
> but this is for a later project!

That's a whole 'nother rabbit hole, search this list (and elsewhere) for
"phase center".  These days even the professional geodesic antennas are
mostly using patches.  The only Helix antennas you can still get seem to
be wherever the original SAGEM have ended up getting owned.

> @ Achim.  Yes indeed, I encounter the problem that my measurement
> resolution is only 100ns. So it takes ages until the frequency locks
> and it takes longer and longer, the closer the frequency is... and I
> exactly assumed what you have pointed out: perhaps my steering becomes
> so sparse in time, that the OCXO's aging rate is faster.  2e-11 at 1s
> is not bad, but hoped for something with e-12 or even better - I think
> this should be possible, not?

Yes.  Even keeping everything the same as you have now, if you get
better resolution on your phase difference it will imporve the
performance considerably.  What I was trying to tell you is that you can
use the M8T to measure the phase difference to considerably better than
100ns and that you can (in addition, if your module has both EXTINT
inputs available) use a divided-down frequency input to the module to
more closely align its internal oscillator to your superior OCXO from
the start.  You can still add a TIC to go into sub-ns territory later
on.

> So I think I would really like to go the PLL route, I hope to achieve
> faster lock time and a bonus would be that the PPS pulses derived from
> my OCXO would be aligned to those of the GPS module.  But how to
> interface the PFD to a microcontroller and implement the loop filter
> is still a mystery for me :-)

You'll end up with some FLL/PLL hybrid most likely.  Anyway, start with
getting better PD measurements and take it from there.


Regards,
Achim.
--
+<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+

Factory and User Sound Singles for Waldorf Q+, Q and microQ:
http://Synth.Stromeko.net/Downloads.html#WaldorfSounds

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Re: [time-nuts] Yet another GPSDO project.

[Tobias Pluess]

Hi again Bob,

I read some documentation about your TICC and tried to figure out how I could 
implement something similar in my GPSDO, but having only one channel. I ended 
up with this:

https://hb9fsx.ch/wordpress/wp-content/uploads/2019/03/20190320_214120.jpg

It works as follows. The GPS 1PPS is asynchronous to the 10MHz clock. So it 
occurs anywhere. It is synced to the 10MHz and delayed by 300ns, the same as in 
the TICC. The rising edge of the 1PPS is used as the start signal for the 
TDC7200, and the rising edge of the delayed 1PPS is used as the stop signal.

While all this happens, the counter inside the microcontroller counts, and is 
latched when the microcontroller detects the rising edge.

On the next 1PPS signal from the GPS the same things happen again, and and the 
counter is latched another time.
In my example, the counter counts from 00 to 08, and the TDC7200 gives the 
additional fractional 70ns.
What do you think?

It needs not 2 TDCs but only one, but it also requires quite a bit of external 
glue logic. And I am not sure about the delay of 300ns: if the propagation 
delay of the logic producing this delay is not accurately known, the delay is 
not 300ns but 300ns + some offset, isn't it?


Tobias
HB9FSX


From: time-nuts [time-nuts-boun...@lists.febo.com] on behalf of Bob kb8tq 
[kb...@n1k.org]
Sent: Wednesday, March 20, 2019 14:27
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Yet another GPSDO project.

Hi

Indeed you want a digital “phase detector” of some sort. Your counter on the 
micro
is a phase detector. If you clock it off of the OCXO and read it based on the 
PPS,
what you get is phase. If you never arbitrarily reset the running counter, it 
reads phase
forever and ever (no slips). If you get a micro that lets you phase lock the 
internal clock,
you can come up with a counter that runs in the hundreds of MHz. That gets you 
at least
into the 10’s of ns. An FPGA is another way to get a counter like this.

Since things like counters are stable long term, they make good phase detectors 
in a
case like this. You might “help” a little to get the last few digits (as the 
TICC does). Still
the bulk of what you read is done by a simple counter. Drift is still taken out 
and you
hang on to your phase reference long term.

As a very simple example:

Most micro’s will let you set up a counter so it rolls over at a set point. The 
example
below assumes it counts to some yy….. = one second value and the next count
is zero.

Second by second my counter at the PPS edge reads out:

1204
1212
1223
1232
1231
1245
1259
…..
1325

and on it goes.

Since I never reset the count, it’s off by some weird amount. I really don’t 
care what that is.
I can look at the readings and if the first and last are 10 seconds apart 
that’s phase over 10
seconds. In the example above my phase changed by 1325-1204 = 121 counts.

All the loop needs to do is to “steer” things so the count stays at xxx1204. 
Subtract what I
saw when I first started up from whatever I got just now and keep that at zero.

Yes this all *assumes* the counter has been set up to roll over nicely at the 
one second
transition. If not, there is a bit more math to do to account for that issue. 
There are also
gotcha’s with counter / timer’s on micro’s maybe not doing quite what you think 
they should
due to clock domain issues.

Some designs out there run a very small counter ( = ignore all the  in the 
example above)
and *assume* that things will never roll over. There are lots of alternatives …

Bob



> On Mar 20, 2019, at 3:53 AM, Tobias Pluess  wrote:
>
> Hi,
>
> no, I don't want the OCXO to closely follow the 1PPS - this would make no 
> sense. Instead I thought the "right way" of doing it is perhaps:
>
> a) measure the phase difference between the GPS PPS and the PPS derived from 
> the OCXO.
> b) put that phase difference into a loop filter.
> c) the loop filter's output goes to a DAC.
>
> Of course I don't want to have an analog phase detector, nor an analog loop 
> filter due to the reasons you already mentioned (small phase detector output 
> voltage, insance capacitor values etc.). Instead both the loop filter and the 
> phase detector should be digital, and this is my current step where I 
> struggled - how to do that.
> But thanks to the loop filter I would expect the phase jitter of the GPS PPS 
> to be averaged out. Of course the loop filter's averaging time should be 
> chosen such that it lies approximately at the point there the ADEV of the 
> OCXO becomes worse than the ADEV of the GPS (however, I have no ADEV info 
> available about my oscillator, *sigh*).
>
> But for example, if my OCXO's ADEV begins to increase at, say 1000s, the 
> perhaps easiest loop filter would be a FIR filter which averages the last 
> 1000 measurements of the phase difference between GPS PPS and OCXO PPS, 
> right? (afterwards one 

Re: [time-nuts] Yet another GPSDO project.

[Tobias Pluess]

Hi Bernd,

awesome that you still remember this :-)
sure, I believe you since my primary requirement was phase noise. This is 
because I'd like to use the OCXO as reference for my spectrum analyzer and also 
for my HP 8663 signal generator to do phase noise measurements - the signal 
generator can be improved quite a bit if a proper external reference is used, 
compared to its internal OCXO! (a HP 10811A)

What you mention is very interesting: you said " As it is from an older 
production lot, the aging may meanwhile be much lower than specified." does 
this mean that the aging of an OCXO does improve, even if the OCXO is unpowered 
in its box?

Do you think the stability of the OCXO improves a bit if I use some thermal 
isolation, as I did in my GPSDO, or is it (as Achim points out) perhaps 
contraproductive.

I'll contact you off list for the measurement - indeed this woud be awesome, 
and if you can offer me an even more stable OCXO this would be fantastic!

Best
Tobias
HB9FSX


From: time-nuts [time-nuts-boun...@lists.febo.com] on behalf of Bernd Neubig 
[bneu...@t-online.de]
Sent: Wednesday, March 20, 2019 19:12
To: 'Discussion of precise time and frequency measurement'
Cc: bneu...@axtal.com
Subject: Re: [time-nuts] Yet another GPSDO project.

Hi Tobias,

Indeed the OCXO which you got from us (AXTAL) was an item from stock, which
was not particularly designed for low ADEV. As it is from an older
production lot, the aging may meanwhile be much lower than specified.

If you want, you may ship your OCXO back to us. We will be glad to measure
ADEV. Maybe we can also offer you one with lower ADEV even.
Best regards
Bernd Neubig DK1AG


-Ursprüngliche Nachricht-
Von: time-nuts [mailto:time-nuts-boun...@lists.febo.com] Im Auftrag von
Tobias Pluess
Gesendet: Mittwoch, 20. März 2019 08:54
An: Discussion of precise time and frequency measurement

Betreff: Re: [time-nuts] Yet another GPSDO project.

Hi,

no, I don't want the OCXO to closely follow the 1PPS - this would make no
sense. Instead I thought the "right way" of doing it is perhaps:

a) measure the phase difference between the GPS PPS and the PPS derived from
the OCXO.
b) put that phase difference into a loop filter.
c) the loop filter's output goes to a DAC.

Of course I don't want to have an analog phase detector, nor an analog loop
filter due to the reasons you already mentioned (small phase detector output
voltage, insance capacitor values etc.). Instead both the loop filter and
the phase detector should be digital, and this is my current step where I
struggled - how to do that.
But thanks to the loop filter I would expect the phase jitter of the GPS PPS
to be averaged out. Of course the loop filter's averaging time should be
chosen such that it lies approximately at the point there the ADEV of the
OCXO becomes worse than the ADEV of the GPS (however, I have no ADEV info
available about my oscillator, *sigh*).

But for example, if my OCXO's ADEV begins to increase at, say 1000s, the
perhaps easiest loop filter would be a FIR filter which averages the last
1000 measurements of the phase difference between GPS PPS and OCXO PPS,
right? (afterwards one could improve this to IIR or whatever to save filter
taps...).

The OCXO I have is perhaps not the most stable one, but also not a low cost
device. It has an aging of perhaps 1ppb per day and 100ppb per year, but it
is not a double oven device, unfortunately... perhaps I should look for an
Oscilloquartz 8663 or so?


Tobias
HB9FSX



From: time-nuts [time-nuts-boun...@lists.febo.com] on behalf of Bob kb8tq
[kb...@n1k.org]
Sent: Tuesday, March 19, 2019 23:35
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Yet another GPSDO project.

Hi

Ok, *if* you phase lock the OCXO to closely follow the GPS PPPS, the OCXO
will be every bit as noisy as the GPS. Actually it will be worse since you
now have 1Hz noise running around that needs to be taken care of ( = the
output of the phase detector is a series of pulses at a 1 Hz rate).

Net result is that you still need a a long time constant loop to do any
good. There also will be some interesting “analog” issues in trying to deal
with the ouput of a normal phase detector at this slow a rate (leakage,
giant capacitors, weird noise issues …).  Bottom line - it’s going to be an
exciting design in a number of ways.

You might say - up the output rate of the GPS. That’s fine, but the
correction information you need only comes out once a second. The rest of
the time you are running with (noisy) uncorrected pulses.  You took one step
forward and two back ….



The normal way the lockup issue is handled is via software and by “cheating”
a bit while the device locks up. Just how you do this depends a *lot* on how
your system is set up. With a pure PLL you don’t have quite as many choices
as with a phase sampling ( TICC counter-like) setup. With phase 

Re: [time-nuts] Yet another GPSDO project.

[Bernd Neubig]

Hi Tobias,

Indeed the OCXO which you got from us (AXTAL) was an item from stock, which
was not particularly designed for low ADEV. As it is from an older
production lot, the aging may meanwhile be much lower than specified.

If you want, you may ship your OCXO back to us. We will be glad to measure
ADEV. Maybe we can also offer you one with lower ADEV even.
Best regards
Bernd Neubig DK1AG


-Ursprüngliche Nachricht-
Von: time-nuts [mailto:time-nuts-boun...@lists.febo.com] Im Auftrag von
Tobias Pluess
Gesendet: Mittwoch, 20. März 2019 08:54
An: Discussion of precise time and frequency measurement

Betreff: Re: [time-nuts] Yet another GPSDO project.

Hi,

no, I don't want the OCXO to closely follow the 1PPS - this would make no
sense. Instead I thought the "right way" of doing it is perhaps:

a) measure the phase difference between the GPS PPS and the PPS derived from
the OCXO.
b) put that phase difference into a loop filter.
c) the loop filter's output goes to a DAC.

Of course I don't want to have an analog phase detector, nor an analog loop
filter due to the reasons you already mentioned (small phase detector output
voltage, insance capacitor values etc.). Instead both the loop filter and
the phase detector should be digital, and this is my current step where I
struggled - how to do that.
But thanks to the loop filter I would expect the phase jitter of the GPS PPS
to be averaged out. Of course the loop filter's averaging time should be
chosen such that it lies approximately at the point there the ADEV of the
OCXO becomes worse than the ADEV of the GPS (however, I have no ADEV info
available about my oscillator, *sigh*).

But for example, if my OCXO's ADEV begins to increase at, say 1000s, the
perhaps easiest loop filter would be a FIR filter which averages the last
1000 measurements of the phase difference between GPS PPS and OCXO PPS,
right? (afterwards one could improve this to IIR or whatever to save filter
taps...).

The OCXO I have is perhaps not the most stable one, but also not a low cost
device. It has an aging of perhaps 1ppb per day and 100ppb per year, but it
is not a double oven device, unfortunately... perhaps I should look for an
Oscilloquartz 8663 or so?


Tobias
HB9FSX



From: time-nuts [time-nuts-boun...@lists.febo.com] on behalf of Bob kb8tq
[kb...@n1k.org]
Sent: Tuesday, March 19, 2019 23:35
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Yet another GPSDO project.

Hi

Ok, *if* you phase lock the OCXO to closely follow the GPS PPPS, the OCXO
will be every bit as noisy as the GPS. Actually it will be worse since you
now have 1Hz noise running around that needs to be taken care of ( = the
output of the phase detector is a series of pulses at a 1 Hz rate).

Net result is that you still need a a long time constant loop to do any
good. There also will be some interesting “analog” issues in trying to deal
with the ouput of a normal phase detector at this slow a rate (leakage,
giant capacitors, weird noise issues …).  Bottom line - it’s going to be an
exciting design in a number of ways.

You might say - up the output rate of the GPS. That’s fine, but the
correction information you need only comes out once a second. The rest of
the time you are running with (noisy) uncorrected pulses.  You took one step
forward and two back ….



The normal way the lockup issue is handled is via software and by “cheating”
a bit while the device locks up. Just how you do this depends a *lot* on how
your system is set up. With a pure PLL you don’t have quite as many choices
as with a phase sampling ( TICC counter-like) setup. With phase samples, you
can look at frequency and lock to that early on. Then switch to a true phase
lock later on. Most high performance GPSDO’s go through multiple steps
before they are in their final lock condition.



Stepping back a bit - Whats good?

A $100 telecom Rb off of eBay will hold 0.1 ppb for a very long time with
very little help. Yes you might buy two or three to get one good one. You
also can do a bit of digging and improve the odds. Second to second wise
they should hold 0.01 ppb.

An fairly good ( = double oven) OCXO might hold 1 ppb for a month after it
warms up for a while. If you buy it on eBay. Second to second wise it should
be around 0.002 ppb

A low cost OCXO might struggle at the 50 ppb level over a month. Second to
second wise ….
who knows … maybe 0.1 ppb (yikes …).

A GSPDO (while locked) will have zero net frequency error (it’s phase
locked).  It’s short term frequency can / will / yikes wander. The net will
be zero. Second to second wise it should be about the same as it’s “core”
device (Rb or OCXO).

Based on all this, I’d suggest that a GPSDO needs to be in the < 0.1 ppb
range in terms of “how good" to beat out the unlocked Rb.

0.1 ppb is a nanosecond over 10 seconds. It is a phase error of 360 /
1,000,000,000 degrees at 1 Hz over that time period. That’s 

Re: [time-nuts] Yet another GPSDO project.

[Achim Gratz]

Tobias Pluess writes:
> By the way, this is the unit I built:
>
> https://hb9fsx.ch/wordpress/index.php/2018/11/14/my-own-gpsdo-part-i/

Nice build.  Minor nit: Whatever foam you used to isolate your OCXO
further is probably not SAGEX, but looks to be either cross-linked PE
foam or (more likely) PU foam.  Isolating the unit like that shifts down
the maximum usable ambient temperature and may confuse the temperature
controller if you overdo it.

> I have also a homemade antenna for it, since I read somewhere that the
> quadrifilar helix antennas are better than patch antennas, I made one
> by myself. A choke ring antenna would perhaps be even more awesome,
> but this is for a later project!

That's a whole 'nother rabbit hole, search this list (and elsewhere) for
"phase center".  These days even the professional geodesic antennas are
mostly using patches.  The only Helix antennas you can still get seem to
be wherever the original SAGEM have ended up getting owned.

> @ Achim.  Yes indeed, I encounter the problem that my measurement
> resolution is only 100ns. So it takes ages until the frequency locks
> and it takes longer and longer, the closer the frequency is... and I
> exactly assumed what you have pointed out: perhaps my steering becomes
> so sparse in time, that the OCXO's aging rate is faster.  2e-11 at 1s
> is not bad, but hoped for something with e-12 or even better - I think
> this should be possible, not?

Yes.  Even keeping everything the same as you have now, if you get
better resolution on your phase difference it will imporve the
performance considerably.  What I was trying to tell you is that you can
use the M8T to measure the phase difference to considerably better than
100ns and that you can (in addition, if your module has both EXTINT
inputs available) use a divided-down frequency input to the module to
more closely align its internal oscillator to your superior OCXO from
the start.  You can still add a TIC to go into sub-ns territory later
on.

> So I think I would really like to go the PLL route, I hope to achieve
> faster lock time and a bonus would be that the PPS pulses derived from
> my OCXO would be aligned to those of the GPS module.  But how to
> interface the PFD to a microcontroller and implement the loop filter
> is still a mystery for me :-)

You'll end up with some FLL/PLL hybrid most likely.  Anyway, start with
getting better PD measurements and take it from there.


Regards,
Achim.
-- 
+<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+

Factory and User Sound Singles for Waldorf Q+, Q and microQ:
http://Synth.Stromeko.net/Downloads.html#WaldorfSounds

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Re: [time-nuts] Yet another GPSDO project.

[Bob kb8tq]

Hi

Indeed you want a digital “phase detector” of some sort. Your counter on the 
micro 
is a phase detector. If you clock it off of the OCXO and read it based on the 
PPS,
what you get is phase. If you never arbitrarily reset the running counter, it 
reads phase 
forever and ever (no slips). If you get a micro that lets you phase lock the 
internal clock, 
you can come up with a counter that runs in the hundreds of MHz. That gets you 
at least
into the 10’s of ns. An FPGA is another way to get a counter like this. 

Since things like counters are stable long term, they make good phase detectors 
in a
case like this. You might “help” a little to get the last few digits (as the 
TICC does). Still
the bulk of what you read is done by a simple counter. Drift is still taken out 
and you
hang on to your phase reference long term. 

As a very simple example:

Most micro’s will let you set up a counter so it rolls over at a set point. The 
example
below assumes it counts to some yy….. = one second value and the next count
is zero. 

Second by second my counter at the PPS edge reads out:

1204
1212
1223
1232
1231
1245
1259
…..
1325

and on it goes. 

Since I never reset the count, it’s off by some weird amount. I really don’t 
care what that is.
I can look at the readings and if the first and last are 10 seconds apart 
that’s phase over 10
seconds. In the example above my phase changed by 1325-1204 = 121 counts. 

All the loop needs to do is to “steer” things so the count stays at xxx1204. 
Subtract what I
saw when I first started up from whatever I got just now and keep that at zero. 

Yes this all *assumes* the counter has been set up to roll over nicely at the 
one second 
transition. If not, there is a bit more math to do to account for that issue. 
There are also 
gotcha’s with counter / timer’s on micro’s maybe not doing quite what you think 
they should
due to clock domain issues. 

Some designs out there run a very small counter ( = ignore all the  in the 
example above) 
and *assume* that things will never roll over. There are lots of alternatives …

Bob



> On Mar 20, 2019, at 3:53 AM, Tobias Pluess  wrote:
> 
> Hi,
> 
> no, I don't want the OCXO to closely follow the 1PPS - this would make no 
> sense. Instead I thought the "right way" of doing it is perhaps:
> 
> a) measure the phase difference between the GPS PPS and the PPS derived from 
> the OCXO.
> b) put that phase difference into a loop filter.
> c) the loop filter's output goes to a DAC.
> 
> Of course I don't want to have an analog phase detector, nor an analog loop 
> filter due to the reasons you already mentioned (small phase detector output 
> voltage, insance capacitor values etc.). Instead both the loop filter and the 
> phase detector should be digital, and this is my current step where I 
> struggled - how to do that.
> But thanks to the loop filter I would expect the phase jitter of the GPS PPS 
> to be averaged out. Of course the loop filter's averaging time should be 
> chosen such that it lies approximately at the point there the ADEV of the 
> OCXO becomes worse than the ADEV of the GPS (however, I have no ADEV info 
> available about my oscillator, *sigh*).
> 
> But for example, if my OCXO's ADEV begins to increase at, say 1000s, the 
> perhaps easiest loop filter would be a FIR filter which averages the last 
> 1000 measurements of the phase difference between GPS PPS and OCXO PPS, 
> right? (afterwards one could improve this to IIR or whatever to save filter 
> taps...).
> 
> The OCXO I have is perhaps not the most stable one, but also not a low cost 
> device. It has an aging of perhaps 1ppb per day and 100ppb per year, but it 
> is not a double oven device, unfortunately... perhaps I should look for an 
> Oscilloquartz 8663 or so?
> 
> 
> Tobias
> HB9FSX
> 
> 
> 
> From: time-nuts [time-nuts-boun...@lists.febo.com] on behalf of Bob kb8tq 
> [kb...@n1k.org]
> Sent: Tuesday, March 19, 2019 23:35
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Yet another GPSDO project.
> 
> Hi
> 
> Ok, *if* you phase lock the OCXO to closely follow the GPS PPPS, the OCXO 
> will be every bit as noisy
> as the GPS. Actually it will be worse since you now have 1Hz noise running 
> around that needs to be
> taken care of ( = the output of the phase detector is a series of pulses at a 
> 1 Hz rate).
> 
> Net result is that you still need a a long time constant loop to do any good. 
> There also will
> be some interesting “analog” issues in trying to deal with the ouput of a 
> normal phase detector at this
> slow a rate (leakage, giant capacitors, weird noise issues …).  Bottom line - 
> it’s going to be an
> exciting design in a number of ways.
> 
> You might say - up the output rate of the GPS. That’s fine, but the 
> correction information you need only
> comes out once a second. The rest of the time you are running with 

Re: [time-nuts] Frequency Ensemble

[Michael Wouters]

Oops, units for TDEV are seconds, not nanoseconds!

On Wed, 20 Mar 2019 at 7:41 pm, Michael Wouters 
wrote:

> Hello Dan,
>
>
> As promised here is a comparison of the ZED-F9P and NEO-M8T
> sawtooth-corrected PPS. The PPS is measured against a 5071 with standard
> tube. Three days of data were used. The 5071 was measured against another
> 5071 and I divided the TDEV by sqrt(2). The 5071 data at shorter than 300s
> is limited by the counter resolution so I didn't show it. The ZED-F9P is
> significantly better than the M8T between 10 and 1000 s but much the same
> past 1s.
>
> Cheers
> Michael
>
> On Tue, 19 Mar 2019 at 8:02 am, Dan Kemppainen 
> wrote:
>
>> Bob,
>>
>> I recalled the plot you posted a few days ago. Nice plot on the F9P,
>> BTW. Thanks for posting that.
>>
>> I don't supposed you happened to be recording a similar plot with a L1
>> only Ublox part at the same time by chance, did you? I think it would be
>> interesting to compare a 6T or M8 part to the F9P. (I lack a standard
>> good enough to compare them to...)
>>
>> Thanks!
>> Dan
>>
>>
>>
>>
>> On 3/18/2019 1:00 PM, time-nuts-requ...@lists.febo.com wrote:
>> > Message: 2
>> > Date: Mon, 18 Mar 2019 12:26:55 -0400
>> > From: Bob kb8tq
>> > To: Discussion of precise time and frequency measurement
>> >   
>> > Subject: Re: [time-nuts] Frequency Ensemble
>> > Message-ID:<88460ea2-6034-48d1-a77a-fed1f824d...@n1k.org>
>> > Content-Type: text/plain; charset=utf-8
>> >
>> > Hi
>> >
>> > There are a variety of GPS devices that put out a PPS. uBlox makes
>> some, there
>> > are a number of other companies that do so. The PPS comes out modulo
>> the local
>> > timebase. On a precision part, there is a ?sawtooth correction? message
>> that also
>> > comes out to further quantify the best guess time of that pulse.
>> >
>> > Noise wise, even with correction you are lucky to get a one second ADEV
>> in the
>> > 1 to 2 ppb range with a typical L1 receiver. With a L1 / L2 device like
>> the F9P,
>> > you might do a bit better than that.
>> >
>> > The ADEV of your other sources at short tau will be much better than
>> the GPS PPS
>> > noise. As you average out over long periods, the GPS will win the race.
>> >
>> > Bob
>>
>> ___
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>> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
>> and follow the instructions there.
>>
>
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Re: [time-nuts] Frequency Ensemble

[Michael Wouters]

Hello Dan,

As promised here is a comparison of the ZED-F9P and NEO-M8T
sawtooth-corrected PPS. The PPS is measured against a 5071 with standard
tube. Three days of data were used. The 5071 was measured against another
5071 and I divided the TDEV by sqrt(2). The 5071 data at shorter than 300s
is limited by the counter resolution so I didn't show it. The ZED-F9P is
significantly better than the M8T between 10 and 1000 s but much the same
past 1s.

Cheers
Michael

On Tue, 19 Mar 2019 at 8:02 am, Dan Kemppainen 
wrote:

> Bob,
>
> I recalled the plot you posted a few days ago. Nice plot on the F9P,
> BTW. Thanks for posting that.
>
> I don't supposed you happened to be recording a similar plot with a L1
> only Ublox part at the same time by chance, did you? I think it would be
> interesting to compare a 6T or M8 part to the F9P. (I lack a standard
> good enough to compare them to...)
>
> Thanks!
> Dan
>
>
>
>
> On 3/18/2019 1:00 PM, time-nuts-requ...@lists.febo.com wrote:
> > Message: 2
> > Date: Mon, 18 Mar 2019 12:26:55 -0400
> > From: Bob kb8tq
> > To: Discussion of precise time and frequency measurement
> >   
> > Subject: Re: [time-nuts] Frequency Ensemble
> > Message-ID:<88460ea2-6034-48d1-a77a-fed1f824d...@n1k.org>
> > Content-Type: text/plain; charset=utf-8
> >
> > Hi
> >
> > There are a variety of GPS devices that put out a PPS. uBlox makes some,
> there
> > are a number of other companies that do so. The PPS comes out modulo the
> local
> > timebase. On a precision part, there is a ?sawtooth correction? message
> that also
> > comes out to further quantify the best guess time of that pulse.
> >
> > Noise wise, even with correction you are lucky to get a one second ADEV
> in the
> > 1 to 2 ppb range with a typical L1 receiver. With a L1 / L2 device like
> the F9P,
> > you might do a bit better than that.
> >
> > The ADEV of your other sources at short tau will be much better than the
> GPS PPS
> > noise. As you average out over long periods, the GPS will win the race.
> >
> > Bob
>
> ___
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> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
> and follow the instructions there.
>
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Re: [time-nuts] Yet another GPSDO project.

[Tobias Pluess]

Hi,

no, I don't want the OCXO to closely follow the 1PPS - this would make no 
sense. Instead I thought the "right way" of doing it is perhaps:

a) measure the phase difference between the GPS PPS and the PPS derived from 
the OCXO.
b) put that phase difference into a loop filter.
c) the loop filter's output goes to a DAC.

Of course I don't want to have an analog phase detector, nor an analog loop 
filter due to the reasons you already mentioned (small phase detector output 
voltage, insance capacitor values etc.). Instead both the loop filter and the 
phase detector should be digital, and this is my current step where I struggled 
- how to do that.
But thanks to the loop filter I would expect the phase jitter of the GPS PPS to 
be averaged out. Of course the loop filter's averaging time should be chosen 
such that it lies approximately at the point there the ADEV of the OCXO becomes 
worse than the ADEV of the GPS (however, I have no ADEV info available about my 
oscillator, *sigh*).

But for example, if my OCXO's ADEV begins to increase at, say 1000s, the 
perhaps easiest loop filter would be a FIR filter which averages the last 1000 
measurements of the phase difference between GPS PPS and OCXO PPS, right? 
(afterwards one could improve this to IIR or whatever to save filter taps...).

The OCXO I have is perhaps not the most stable one, but also not a low cost 
device. It has an aging of perhaps 1ppb per day and 100ppb per year, but it is 
not a double oven device, unfortunately... perhaps I should look for an 
Oscilloquartz 8663 or so?


Tobias
HB9FSX



From: time-nuts [time-nuts-boun...@lists.febo.com] on behalf of Bob kb8tq 
[kb...@n1k.org]
Sent: Tuesday, March 19, 2019 23:35
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Yet another GPSDO project.

Hi

Ok, *if* you phase lock the OCXO to closely follow the GPS PPPS, the OCXO will 
be every bit as noisy
as the GPS. Actually it will be worse since you now have 1Hz noise running 
around that needs to be
taken care of ( = the output of the phase detector is a series of pulses at a 1 
Hz rate).

Net result is that you still need a a long time constant loop to do any good. 
There also will
be some interesting “analog” issues in trying to deal with the ouput of a 
normal phase detector at this
slow a rate (leakage, giant capacitors, weird noise issues …).  Bottom line - 
it’s going to be an
exciting design in a number of ways.

You might say - up the output rate of the GPS. That’s fine, but the correction 
information you need only
comes out once a second. The rest of the time you are running with (noisy) 
uncorrected pulses.  You
took one step forward and two back ….



The normal way the lockup issue is handled is via software and by “cheating” a 
bit while the device
locks up. Just how you do this depends a *lot* on how your system is set up. 
With a pure PLL you
don’t have quite as many choices as with a phase sampling ( TICC counter-like) 
setup. With phase samples,
you can look at frequency and lock to that early on. Then switch to a true 
phase lock later on. Most
high performance GPSDO’s go through multiple steps before they are in their 
final lock condition.



Stepping back a bit - Whats good?

A $100 telecom Rb off of eBay will hold 0.1 ppb for a very long time with very 
little help. Yes you might
buy two or three to get one good one. You also can do a bit of digging and 
improve the odds. Second
to second wise they should hold 0.01 ppb.

An fairly good ( = double oven) OCXO might hold 1 ppb for a month after it 
warms up for a
while. If you buy it on eBay. Second to second wise it should be around 0.002 
ppb

A low cost OCXO might struggle at the 50 ppb level over a month. Second to 
second wise ….
who knows … maybe 0.1 ppb (yikes …).

A GSPDO (while locked) will have zero net frequency error (it’s phase locked).  
It’s short
term frequency can / will / yikes wander. The net will be zero. Second to 
second wise it should
be about the same as it’s “core” device (Rb or OCXO).

Based on all this, I’d suggest that a GPSDO needs to be in the < 0.1 ppb range 
in terms of
“how good" to beat out the unlocked Rb.

0.1 ppb is a nanosecond over 10 seconds. It is a phase error of 360 / 
1,000,000,000 degrees
at 1 Hz over that time period. That’s a pretty small phase change and not a 
very big time change.
At 100 seconds you are up to 10 ns (so a bit easier to spot). You are also at 
10X the degrees,
still pretty tough.

To turn the phase into other units:  if your phase detector has a 1V / radian 
slope (not uncommon on
an analog phase detector), you are after 2*PI / 1,000,000,000 volts on the 
output. That’s just a bit over
6 nV. Ideally you would want your detector drift to be < 20% of this (or about 
1 nV) to get the job done
…. yikes ….

Lots of fun !!

Bob

> On Mar 19, 2019, at 4:43 PM, Tobias Pluess  wrote:
>
> Thanks for the replies so far.
>